The goal of this blog entry is
to describe
how you can
create custom data sources which you can use with the controls in the WinJS library. In particular, I explain
how you can
create an IndexedDbDataSource which you can use
to store and retrieve data from
an IndexedDB database.
If you want
to skip ahead, and ignore all of the fascinating content in-between, I’ve included the complete code for the IndexedDbDataSource at the very bottom of this blog entry.
What is IndexedDB?
IndexedDB is a database in the browser. You can use the IndexedDB API with all modern browsers including Firefox, Chrome, and Internet Explorer 10. And, of course, you can use IndexedDB with Metro style apps written with JavaScript.
If you need
to persist data in a Metro style app written with JavaScript then IndexedDB is a good option. Each Metro app can only interact with its own IndexedDB databases. And, IndexedDB provides you with transactions, indices, and cursors – the elements of any modern database.
An IndexedDB database might be different than the type of database that you normally use.
An IndexedDB database is
an object-oriented database and not a relational database. Instead of storing data in tables, you store data in object stores. You store JavaScript objects in
an IndexedDB object store.
You
create new IndexedDB object stores by handling the upgradeneeded event when you attempt
to open a connection
to an IndexedDB database. For example, here’s
how you would both open a connection
to an existing database named TasksDB and
create the TasksDB database when it does not already exist:
var reqOpen = window.indexedDB.open(“TasksDB”, 2);
reqOpen.onupgradeneeded = function (evt) {
var newDB = evt.target.result;
newDB.createObjectStore("tasks", { keyPath: "id", autoIncrement: true });
};
reqOpen.onsuccess = function () {
var db = reqOpen.result;
// Do something with db
};
When you call window.indexedDB.open(), and the database does not already exist, then the upgradeneeded event is raised. In the code above, the upgradeneeded handler creates a new object store named tasks. The new object store has
an auto-increment column named id which acts as the primary key column.
If the database already exists with the right version, and you call window.indexedDB.open(), then the success event is raised. At that point, you have
an open connection
to the existing database and you can start doing something with the database.
You use asynchronous methods
to interact with
an IndexedDB database. For example, the following code illustrates
how you would add a new object
to the tasks object store:
var transaction = db.transaction(“tasks”, “readwrite”);
var reqAdd = transaction.objectStore(“tasks”).add({
name: “Feed the dog”
});
reqAdd.onsuccess = function() {
// Tasks added successfully
};
The code above creates a new database transaction, adds a new task
to the tasks object store, and handles the success event. If the new task gets added successfully then the success event is raised.
Creating a WinJS IndexedDbDataSource
The most powerful control in the WinJS library is the ListView control. This is the control that you use
to display a collection of items.
If you want
to display data with a ListView control, you need
to bind the control
to a data source. The WinJS library includes two objects which you can use as a data source: the List object and the StorageDataSource object. The List object enables you
to represent a JavaScript array as a data source and the StorageDataSource enables you
to represent the file system as a data source.
If you want
to bind
an IndexedDB database
to a ListView then you have a choice. You can either dump the items from the IndexedDB database into a List object or you can
create a custom data source. I explored the first approach in a previous blog entry. In this blog entry, I explain
how you can
create a custom IndexedDB data source.
Implementing the IListDataSource Interface
You
create a custom data source by implementing the IListDataSource interface. This interface contains the contract for the methods which the ListView needs
to interact with a data source.
The easiest way
to implement the IListDataSource interface is
to derive a new object from the base VirtualizedDataSource object. The VirtualizedDataSource object requires a data adapter which implements the IListDataAdapter interface.
Yes, because of the number of objects involved, this is a little confusing. Your code ends up looking something like this:
var IndexedDbDataSource = WinJS.Class.derive(
WinJS.UI.VirtualizedDataSource,
function (dbName, dbVersion, objectStoreName, upgrade, error) {
this._adapter = new IndexedDbDataAdapter(dbName, dbVersion, objectStoreName, upgrade, error);
this._baseDataSourceConstructor(this._adapter);
},
{
nuke: function () {
this._adapter.nuke();
},
remove: function (key) {
this._adapter.removeInternal(key);
}
}
);
The code above is used
to create a new class named IndexedDbDataSource which derives from the base VirtualizedDataSource class. In the constructor for the new class, the base class _baseDataSourceConstructor() method is called. A data adapter is passed
to the _baseDataSourceConstructor() method.
The code above creates a new method exposed by the IndexedDbDataSource named nuke(). The nuke() method deletes all of the objects from
an object store. The code above also overrides a method named remove(). Our derived remove() method accepts any type of key and removes the matching item from the object store.
Almost all of the work of creating a custom data source goes into building the data adapter class. The data adapter class implements the IListDataAdapter interface which contains the following methods:
· change()
· getCount()
· insertAfter()
· insertAtEnd()
· insertAtStart()
· insertBefore()
· itemsFromDescription()
· itemsFromEnd()
· itemsFromIndex()
· itemsFromKey()
· itemsFromStart()
· itemSignature()
· moveAfter()
· moveBefore()
· moveToEnd()
· moveToStart()
· remove()
· setNotificationHandler()
· compareByIdentity
Fortunately, you are not required
to implement all of these methods. You only need
to implement the methods that you actually need.
In the case of the IndexedDbDataSource, I implemented the getCount(), itemsFromIndex(), insertAtEnd(), and remove() methods. If you are creating a read-only data source then you really only need
to implement the getCount() and itemsFromIndex() methods.
Implementing the getCount() Method
The getCount() method returns the total number of items from the data source. So, if you are storing 10,000 items in
an object store then this method would return the value 10,000.
Here’s
how I implemented the getCount() method:
getCount: function () {
var that = this;
return new WinJS.Promise(function (complete, error) {
that._getObjectStore().then(function (store) {
var reqCount = store.count();
reqCount.onerror = that._error;
reqCount.onsuccess = function (evt) {
complete(evt.target.result);
};
});
});
}
The first thing that you should notice is that the getCount() method returns a WinJS promise. This is a requirement. The getCount() method is asynchronous which is a good thing because all of the IndexedDB methods (at least the methods implemented in current browsers) are also asynchronous.
The code above retrieves
an object store and then uses the IndexedDB count() method
to get a count of the items in the object store. The value is returned from the promise by calling complete().
Implementing the itemsFromIndex method
When a ListView displays its items, it calls the itemsFromIndex() method. By default, it calls this method multiple times
to get different ranges of items.
Three parameters are passed
to the itemsFromIndex() method: the requestIndex, countBefore, and countAfter parameters. The requestIndex indicates the index of the item from the database
to show. The countBefore and countAfter parameters represent hints. These are integer values which represent the number of items before and after the requestIndex
to retrieve. Again, these are only hints and you can return as many items before and after the request index as you please.
Here’s
how I implemented the itemsFromIndex method:
itemsFromIndex: function (requestIndex, countBefore, countAfter) {
var that = this;
return new WinJS.Promise(function (complete, error) {
that.getCount().then(function (count) {
if (requestIndex >= count) {
return WinJS.Promise.wrapError(new WinJS.ErrorFromName(WinJS.UI.FetchError.doesNotExist));
}
var startIndex = Math.max(0, requestIndex - countBefore);
var endIndex = Math.min(count, requestIndex + countAfter + 1);
that._getObjectStore().then(function (store) {
var index = 0;
var items = [];
var req = store.openCursor();
req.onerror = that._error;
req.onsuccess = function (evt) {
var cursor = evt.target.result;
if (index < startIndex) {
index = startIndex;
cursor.advance(startIndex);
return;
}
if (cursor && index < endIndex) {
index++;
items.push({
key: cursor.value[store.keyPath].toString(),
data: cursor.value
});
cursor.continue();
return;
}
results = {
items: items,
offset: requestIndex - startIndex,
totalCount: count
};
complete(results);
};
});
});
});
}
In the code above, a cursor is used
to iterate through the objects in
an object store. You fetch the next item in the cursor by calling either the cursor.continue() or cursor.advance() method. The continue() method moves forward by one object and the advance() method moves forward a specified number of objects.
Each time you call continue() or advance(), the success event is raised again. If the cursor is null then you know that you have reached the end of the cursor and you can return the results.
Some things
to be careful about here. First, the return value from the itemsFromIndex() method must implement the IFetchResult interface. In particular, you must return
an object which has
an items, offset, and totalCount property.
Second, each item in the items array must implement the IListItem interface. Each item should have a key and a data property.
Implementing the insertAtEnd() Method
When creating the IndexedDbDataSource, I wanted
to go beyond creating a simple read-only data source and support inserting and deleting objects. If you want
to support adding new items with your data source then you need
to implement the insertAtEnd() method.
Here’s
how I implemented the insertAtEnd() method for the IndexedDbDataSource:
insertAtEnd:function(unused, data) {
var that = this;
return new WinJS.Promise(function (complete, error) {
that._getObjectStore("readwrite").done(function(store) {
var reqAdd = store.add(data);
reqAdd.onerror = that._error;
reqAdd.onsuccess = function (evt) {
var reqGet = store.get(evt.target.result);
reqGet.onerror = that._error;
reqGet.onsuccess = function (evt) {
var newItem = {
key:evt.target.result[store.keyPath].toString(),
data:evt.target.result
}
complete(newItem);
};
};
});
});
}
When implementing the insertAtEnd() method, you need
to be careful
to return
an object which implements the IItem interface. In particular, you should return
an object that has a key and a data property. The key must be a string and it uniquely represents the new item added
to the data source. The value of the data property represents the new item itself.
Implementing the remove() Method
Finally, you use the remove() method
to remove
an item from the data source. You call the remove() method with the key of the item which you want
to remove.
Implementing the remove() method in the case of the IndexedDbDataSource was a little tricky. The problem is that
an IndexedDB object store uses
an integer key and the VirtualizedDataSource requires a string key. For that reason, I needed
to override the remove() method in the derived IndexedDbDataSource class like this:
var IndexedDbDataSource = WinJS.Class.derive(
WinJS.UI.VirtualizedDataSource,
function (dbName, dbVersion, objectStoreName, upgrade, error) {
this._adapter = new IndexedDbDataAdapter(dbName, dbVersion, objectStoreName, upgrade, error);
this._baseDataSourceConstructor(this._adapter);
},
{
nuke: function () {
this._adapter.nuke();
},
remove: function (key) {
this._adapter.removeInternal(key);
}
}
);
When you call remove(), you end up calling a method of the IndexedDbDataAdapter named removeInternal() . Here’s what the removeInternal() method looks like:
setNotificationHandler: function (notificationHandler) {
this._notificationHandler = notificationHandler;
},
removeInternal: function(key) {
var that = this;
return new WinJS.Promise(function (complete, error) {
that._getObjectStore("readwrite").done(function (store) {
var reqDelete = store.delete (key);
reqDelete.onerror = that._error;
reqDelete.onsuccess = function (evt) {
that._notificationHandler.removed(key.toString());
complete();
};
});
});
}
The removeInternal() method calls the IndexedDB delete() method
to delete
an item from the object store. If the item is deleted successfully then the _notificationHandler.remove() method is called.
Because we are not implementing the standard IListDataAdapter remove() method, we need
to notify the data source (and the ListView control bound
to the data source) that
an item has been removed. The way that you notify the data source is by calling the _notificationHandler.remove() method.
Notice that we get the _notificationHandler in the code above by implementing another method in the IListDataAdapter interface: the setNotificationHandler() method. You can raise the following types of notifications using the _notificationHandler:
· beginNotifications()
· changed()
· endNotifications()
· inserted()
· invalidateAll()
· moved()
· removed()
· reload()
These methods are all part of the IListDataNotificationHandler interface in the WinJS library.
Implementing the nuke() Method
I wanted
to implement a method which would remove all of the items from
an object store. Therefore, I created a method named nuke() which calls the IndexedDB clear() method:
nuke: function () {
var that = this;
return new WinJS.Promise(function (complete, error) {
that._getObjectStore("readwrite").done(function (store) {
var reqClear = store.clear();
reqClear.onerror = that._error;
reqClear.onsuccess = function (evt) {
that._notificationHandler.reload();
complete();
};
});
});
}
Notice that the nuke() method calls the _notificationHandler.reload() method
to notify the ListView
to reload all of the items from its data source. Because we are implementing a custom method here, we need
to use the _notificationHandler
to send
an update.
Using the IndexedDbDataSource
To illustrate
how you can use the IndexedDbDataSource, I created a simple task list app. You can add new tasks, delete existing tasks, and nuke all of the tasks.
You delete
an item by selecting
an item (swipe or right-click) and clicking the Delete button.
Here’s the HTML page which contains the ListView, the form for adding new tasks, and the buttons for deleting and nuking tasks:
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>DataSources</title>
<!-- WinJS references -->
<link href="//Microsoft.WinJS.1.0.RC/css/ui-dark.css" rel="stylesheet" />
<script src="//Microsoft.WinJS.1.0.RC/js/base.js"></script>
<script src="//Microsoft.WinJS.1.0.RC/js/ui.js"></script>
<!-- DataSources references -->
<link href="indexedDb.css" rel="stylesheet" />
<script type="text/javascript" src="indexedDbDataSource.js"></script>
<script src="indexedDb.js"></script>
</head>
<body>
<div id="tmplTask" data-win-control="WinJS.Binding.Template">
<div class="taskItem">
Id: <span data-win-bind="innerText:id"></span>
<br /><br />
Name: <span data-win-bind="innerText:name"></span>
</div>
</div>
<div id="lvTasks"
data-win-control="WinJS.UI.ListView"
data-win-options="{
itemTemplate: select('#tmplTask'),
selectionMode: 'single'
}"></div>
<form id="frmAdd">
<fieldset>
<legend>Add Task</legend>
<label>New Task</label>
<input id="inputTaskName" required />
<button>Add</button>
</fieldset>
</form>
<button id="btnNuke">Nuke</button>
<button id="btnDelete">Delete</button>
</body>
</html>
And here is the JavaScript code for the TaskList app:
/// <reference path="//Microsoft.WinJS.1.0.RC/js/base.js" />
/// <reference path="//Microsoft.WinJS.1.0.RC/js/ui.js" />
function init() {
WinJS.UI.processAll().done(function () {
var lvTasks = document.getElementById("lvTasks").winControl;
// Bind the ListView
to its data source
var tasksDataSource = new DataSources.IndexedDbDataSource("TasksDB", 1, "tasks", upgrade);
lvTasks.itemDataSource = tasksDataSource;
// Wire-up Add, Delete, Nuke buttons
document.getElementById("frmAdd").addEventListener("submit", function (evt) {
evt.preventDefault();
tasksDataSource.beginEdits();
tasksDataSource.insertAtEnd(null, {
name: document.getElementById("inputTaskName").value
}).done(function (newItem) {
tasksDataSource.endEdits();
document.getElementById("frmAdd").reset();
lvTasks.ensureVisible(newItem.index);
});
});
document.getElementById("btnDelete").addEventListener("click", function () {
if (lvTasks.selection.count() == 1) {
lvTasks.selection.getItems().done(function (items) {
tasksDataSource.remove(items[0].data.id);
});
}
});
document.getElementById("btnNuke").addEventListener("click", function () {
tasksDataSource.nuke();
});
// This method is called
to initialize the IndexedDb database
function upgrade(evt) {
var newDB = evt.target.result;
newDB.createObjectStore("tasks", { keyPath: "id", autoIncrement: true });
}
});
}
document.addEventListener("DOMContentLoaded", init);
The IndexedDbDataSource is created and bound
to the ListView control with the following two lines of code:
var tasksDataSource = new DataSources.IndexedDbDataSource("TasksDB", 1, "tasks", upgrade);
lvTasks.itemDataSource = tasksDataSource;
The IndexedDbDataSource is created with four parameters: the name of the database
to create, the version of the database
to create, the name of the object store
to create, and a function which contains code
to initialize the new database.
The upgrade function creates a new object store named tasks with
an auto-increment property named id:
function upgrade(evt) {
var newDB = evt.target.result;
newDB.createObjectStore("tasks", { keyPath: "id", autoIncrement: true });
}
The Complete Code for the IndexedDbDataSource
Here’s the complete code for the IndexedDbDataSource:
(function () {
/************************************************
* The IndexedDBDataAdapter enables you
to work
* with a HTML5 IndexedDB database.
*************************************************/
var IndexedDbDataAdapter = WinJS.Class.define(
function (dbName, dbVersion, objectStoreName, upgrade, error) {
this._dbName = dbName; // database name
this._dbVersion = dbVersion; // database version
this._objectStoreName = objectStoreName; // object store name
this._upgrade = upgrade; // database upgrade script
this._error = error || function (evt) { console.log(evt.message); };
},
{
/*******************************************
* IListDataAdapter Interface Methods
********************************************/
getCount: function () {
var that = this;
return new WinJS.Promise(function (complete, error) {
that._getObjectStore().then(function (store) {
var reqCount = store.count();
reqCount.onerror = that._error;
reqCount.onsuccess = function (evt) {
complete(evt.target.result);
};
});
});
},
itemsFromIndex: function (requestIndex, countBefore, countAfter) {
var that = this;
return new WinJS.Promise(function (complete, error) {
that.getCount().then(function (count) {
if (requestIndex >= count) {
return WinJS.Promise.wrapError(new WinJS.ErrorFromName(WinJS.UI.FetchError.doesNotExist));
}
var startIndex = Math.max(0, requestIndex - countBefore);
var endIndex = Math.min(count, requestIndex + countAfter + 1);
that._getObjectStore().then(function (store) {
var index = 0;
var items = [];
var req = store.openCursor();
req.onerror = that._error;
req.onsuccess = function (evt) {
var cursor = evt.target.result;
if (index < startIndex) {
index = startIndex;
cursor.advance(startIndex);
return;
}
if (cursor && index < endIndex) {
index++;
items.push({
key: cursor.value[store.keyPath].toString(),
data: cursor.value
});
cursor.continue();
return;
}
results = {
items: items,
offset: requestIndex - startIndex,
totalCount: count
};
complete(results);
};
});
});
});
},
insertAtEnd:function(unused, data) {
var that = this;
return new WinJS.Promise(function (complete, error) {
that._getObjectStore("readwrite").done(function(store) {
var reqAdd = store.add(data);
reqAdd.onerror = that._error;
reqAdd.onsuccess = function (evt) {
var reqGet = store.get(evt.target.result);
reqGet.onerror = that._error;
reqGet.onsuccess = function (evt) {
var newItem = {
key:evt.target.result[store.keyPath].toString(),
data:evt.target.result
}
complete(newItem);
};
};
});
});
},
setNotificationHandler: function (notificationHandler) {
this._notificationHandler = notificationHandler;
},
/*****************************************
* IndexedDbDataSource Method
******************************************/
removeInternal: function(key) {
var that = this;
return new WinJS.Promise(function (complete, error) {
that._getObjectStore("readwrite").done(function (store) {
var reqDelete = store.delete (key);
reqDelete.onerror = that._error;
reqDelete.onsuccess = function (evt) {
that._notificationHandler.removed(key.toString());
complete();
};
});
});
},
nuke: function () {
var that = this;
return new WinJS.Promise(function (complete, error) {
that._getObjectStore("readwrite").done(function (store) {
var reqClear = store.clear();
reqClear.onerror = that._error;
reqClear.onsuccess = function (evt) {
that._notificationHandler.reload();
complete();
};
});
});
},
/*******************************************
* Private Methods
********************************************/
_ensureDbOpen: function () {
var that = this;
// Try
to get cached Db
if (that._cachedDb) {
return WinJS.Promise.wrap(that._cachedDb);
}
// Otherwise, open the database
return new WinJS.Promise(function (complete, error, progress) {
var reqOpen = window.indexedDB.open(that._dbName, that._dbVersion);
reqOpen.onerror = function (evt) {
error();
};
reqOpen.onupgradeneeded = function (evt) {
that._upgrade(evt);
that._notificationHandler.invalidateAll();
};
reqOpen.onsuccess = function () {
that._cachedDb = reqOpen.result;
complete(that._cachedDb);
};
});
},
_getObjectStore: function (type) {
type = type || "readonly";
var that = this;
return new WinJS.Promise(function (complete, error) {
that._ensureDbOpen().then(function (db) {
var transaction = db.transaction(that._objectStoreName, type);
complete(transaction.objectStore(that._objectStoreName));
});
});
},
_get: function (key) {
return new WinJS.Promise(function (complete, error) {
that._getObjectStore().done(function (store) {
var reqGet = store.get(key);
reqGet.onerror = that._error;
reqGet.onsuccess = function (item) {
complete(item);
};
});
});
}
}
);
var IndexedDbDataSource = WinJS.Class.derive(
WinJS.UI.VirtualizedDataSource,
function (dbName, dbVersion, objectStoreName, upgrade, error) {
this._adapter = new IndexedDbDataAdapter(dbName, dbVersion, objectStoreName, upgrade, error);
this._baseDataSourceConstructor(this._adapter);
},
{
nuke: function () {
this._adapter.nuke();
},
remove: function (key) {
this._adapter.removeInternal(key);
}
}
);
WinJS.Namespace.define("DataSources", {
IndexedDbDataSource: IndexedDbDataSource
});
})();
Summary
In this blog post, I provided
an overview of
how you can
create a new data source which you can use with the WinJS library. I described
how you can
create an IndexedDbDataSource which you can use
to bind a ListView control
to an IndexedDB database.
While describing
how you can
create a custom data source, I explained
how you can implement the IListDataAdapter interface. You also learned
how to raise notifications — such as a removed or invalidateAll notification — by taking advantage of the methods of the IListDataNotificationHandler interface.
